We stumbled across an Isango Review on TripAdvisor, which speaks for itself.

Question from Tourist:

Isango charge £19.50. for a half day tour.

Could filling a taxi be a better & cheaper option ?

Answer from Local:

The rate would be 60 MAD return per person, but this applies to people who take a taxi from the taxi rank in Agadir’s city center, not from the port. So unless you are willing to take a petit taxi from the port to the city center, then haggle hard to get a half seat for 30 Dirhams, only then this would be feasible. The rate of 60 MAD return each would include an old taxi with no seat-belts or AC, and no foreign language speaking driver of course.

It is just my opinion, but I would not recommend this option.

Reply from Tourist:

Thank you for the information. You have made my mind up & I will go with Isango.